1. Field of the Invention
The present invention relates to a chopper comparator made up of a plurality of logic gates for comparing an input voltage with a reference voltage and for outputting a comparison result.
2. Description of the Related Art
A conventional chopper comparator is made up of a switch, a first inverter amplifier, and a second inverter amplifier. The switch switches between an analogue input voltage and a reference voltage. The first inverter amplifier is connected to the switch through a capacitor. The second inverter amplifier directly receives the output of the first inverter amplifier. Like a conventional technique, for example, disclosed in the Japanese laid open publication number H4-14312 (pages 2-3 and FIG. 1 to FIG. 6), there is a conventional chopper comparator in which a NMOS transistor or a PMOS transistor is used as the second inverter amplifier, having a gate controlled by a clock signal, and the second inverter amplifier halts its operation during a short circuit between the input node and the output node of the first inverter amplifier that occurs based on a clock signal.
Next, a description will now be given of the operation of the conventional chopper comparator.
The first inverter amplifier controls the switch based on the clock signal in order to make a short circuit between the input node and the output node of the first inverter amplifier. While this switch is ON, an analogue input voltage is supplied to the capacitor connected to the input node of the first inverter amplifier. When this switch is OFF, namely, when no short circuit between the input node and the output node of the first inverter amplifier occurs, the capacitor maintains a voltage difference between the analogue input voltage and the threshold voltage of the first inverter amplifier.
Next, the reference voltage is applied to the capacitor, and the first inverter amplifier compares the analogue input voltage inputted through the capacitor with the reference voltage. The first inverter amplifier outputs a voltage according to the difference between the comparison result and the threshold voltage of the first inverter amplifier. For example, when the analogue input voltage is higher than the reference voltage, the first inverter amplifier outputs a voltage of a high level, and when the analogue input voltage is lower than the reference voltage the first inverter amplifier outputs a voltage of a low level.
In such a comparison operation, while the short circuit between the input node and the output node in the first inverter amplifier occurs, an NMOS transistor of the second inverter amplifier prevents a penetrate current from flowing through the second inverter amplifier. This NMOS transistor disconnects and connects the second inverter amplifier, in synchronization with the clock signal, to control the operation of the switch to make the short circuit between the input node and the output node of the first inverter amplifier.
In the conventional chopper amplifier having the configuration described above, because the path between the input node of the logic gate in the input stage and the capacitance becomes a floating node, where a penetrate current may flow through the logic gate in the input stage while no comparison operation between the input voltage and the reference voltage is performed. Further, the penetrate current flows through the inverter circuit in the logic gate in the output stage when the same voltage as the threshold voltage of the inverter circuit in the second inverter amplifier, namely, in the logic gate in the output stage, is applied to the input node of the logic gate in the output stage, in the no-operation condition when the inverter circuit in the logic circuit in the input stage is in the auto-zero state, or when no comparison operation between the input voltage and the reference voltage is performed. Thus, the conventional technique has a drawback that this penetrate current increases the amount of useless current consumption.